A Kite Can't Fly with a Broken String

In chapter 5 of Slaughterhouse-Five, the chapter goes on and on and on for fifty pages with no purpose whatsoever many wild and incredible events occur with Billy Pilgrim. One of these crazy events is found  on pages 96 and 97, when Billy catches on fire after standing too close to a stove. Upon beating out the sparks with his hands without response from Billy, an Englishman comments, "'My God- what have they done to you, lad? This isn't a man. It's a broken kite'" (Vonnegut, 97). Ouch. That hurts. And no, I'm not talking about having one's coat catch on fire. I mean that being metaphorically called a "broken kite" certainly can't make anyone feel particularly great about himself. What the Brit probably means in his metaphor is that due to Billy's unresponsive nature, he assumes that Billy has been tortured or abused so that he is currently mentally gone. (See, "we never assume in geometry", and certainly do not assume in literature either.) The Englishman is stunned to see what has become of this poor Yank. So stunned is he that he later asks, "'Are you really an American?'" (97).
I don't know whether to be proud or offended by that question, but I think he means that he expected much better of Americans than this sore sap of a bump-on-a-log, Billy. Anyways, Billy is metaphorically called a broken kite for this reason: kites can't fly if they're broken; humans can't function without a brain. Vonnegut probably included this metaphor to continue prove that Billy had a really crummy, unbearable experience in the war. Being insulted by his partners in war is yet another notch in The Broken Kite's unfortunate life.